Hum elimination



April 2, 1935. Q J scl-l 1,996,378

HUM ELIMINATION Filed Dec. 21, 1931 3 Sheets-Sheet 1 INVENTOR CHARLES J HIRSCH PM, 1061:, 777% MW.

ATTORNEYS April 2, 1935. c J scH 1,996,378

HUM ELIMINATION I Filed Dec. 21, 1931 3 Sheets-Sheet 2 Ill ' ATTORNEYS April 2, 1935. c. J. HIRSCH 1,996,378

HUM ELIMINATION Filed Dec. 21, 1931 3 Sheets-Sheet 3 V o noun N Q noun BY V 9 PM, AMJWM dazmmat ATTORNEYS Patented Apr. 2, 1935 UNITED STATES- PATENT OFFICE 1 ,996,378

HUMHELIMINATION; Charles'l flirsch, New York, N; Y. 7 Application December 21, 1931, Serial No. 582,360

' 21Claims. "(CLZSQ-ZY) This invention relates to an arrangement for utilizing rectified'alternating current as a direct current source for energizing vacuum tubes. Particularly,- it relates to. radio receivers in which the ordinary home alternatingcurrent supply is rectified to furnish direct'current voltages to the electrodes of the various vacuum tubes of the receiver. In this connection, the presentin'vention effects a reduction in the expensive and heavy parts usually required, resulting in econ only of design.

Heretofore it has been the practice to. rectify alternating current usually by means of thermionic rectifiers and arrange a filter comprising inductance and capacity, for suppressing any alternatingcurrent components. Such practice has-been effective in providing a hum-free source, but is expensivaand so those Working in this art have continued to seek amore simple and inex-" pensive system, as well as one having greater economy in Weight and size. The primary object of the invention is to provide a hum-free path, which includes the various vacuum tubes of a radio receiver, by means ofa bridge network and a thermionic vacuum tube,

whereby it is possible to omit the usual induc-I tance and capacity filter heretofore necessary'to eliminate hum frequency currents present in the output of a rectifier of alternating current. 30

Another object of the invention is to utilize a vacuum tube to provide a hum-free path which includes a radio receiver, in which the ordinary 40 filter to, aid in freeing the path of hum.

The improvements of this invention include eliminating-the usual inductance and capacity filter and substituting therefor a thermionic vac= uum tube arranged in parallel with resistors proportioned in the ratio of [L to 1 (where a is the amplification constant of the tube) to efiect a bridge. By this arrangementall alternating currents are effectively balanced out in a-path from the plate of the tube to the highest positive potential point of the power supply system, and thus provide what is termed a hum-free path. In this path is located the load or work circuit Another object of the invention is to provide a hum-free path which serves as a source of 1 voltages for operating a vacuum tube receiver by the use-of a bridged network and avacuum tube,-

by-pass capacities of the radio receiver, together I with the res'istanceof this vacuum tube, act as a' (generally a radioreceiver) which requires'humfree voltages for operation. l v

In thepreferred form of this invention, in" which thehum-free pathis utilized as a'volt'age supply for operation of a radio receive1", the f exact balanced bridge arrangement previously referred to need not'be strictly applied and may in fact be departed from to aconsiderable degree because of the" fact that the radio receiver" is primarily. a system of cascaded vacuum tubes, in '10 eluding icy-passing capacities across the points where voltages are applied to energize the tubes. The radio receiver, therefore, supplies the capac ity element of a resistance-capacity filter, while the vacuumtube serves as the resistive element, 5' to help smooth out any alternating-current or hum components and so lessens therequirement of exact bridge balance, which theory would otherwise require; An additional economy isobtained' by-utiliz- 20 ing the vacuum tube, which acts to provide the hum-free path, for the additional purpose of acting as one of the amplifying vacuum tubes .of the, radio receiver. V V I The invention furthermore includes other features such as utilizing more than ,one tube to provide a hum-free path and'includes the im portant feature of utilizing two tubes in a push- 1 pull or. balanced arrangement whereby the dual function of acting to eliminate hum and to amplify signals is better accomplished. In this arrangement the amplifying function is effected by the tubes operated'inpush-pull and hence differentially to the operation of the tubes, f'unctioning to eliminate hum.f

- The push-pull arrangement permits the simultaneous functions of hum elimination and amplie fication of signals by two tubes While providing a balance against feed-back from the. output to the input of the radio receiver. Furthermore, hum voltages present in the push-pull arrangement automatically balance out. The tubes thus function as amplifiers free from hum, and the resistance of the tubes iseifectively present as the resistive element of a resistance capacity filter, as before mentioned, thus making unnecessary a perfect bridge balance against hum.

Having thus briefly describedthe invention,"

attentionlis' invited to the drawings, in which:

Fig; 2 shows a circuit diagram of the preferred method of providing a hum -free path;

fierof'alternating current. Terminals I I and I2 Fig.3 is 'a; circuit diagram of an alternative arrangement for providing a hum-free path;

Fig. 4 shows a circuit diagram which is a modification of Fig. 2, wherein a screen-grid type of vacuum tube is employed;

Fig. 5 is a circuit diagram showing, schematically, the method of utilizing a vacuum tube simultaneously to provide a hum-free path to operate a load requiring hum-free voltages, and to function as an amplifier;

Fig. 6 is a circuit diagram similar to Fig. 5, except that a push-pull arrangement of two tubes is shown;

Fig. 7 is a diagram illustrating a complete power supply system in which a single vacuumztube simuletaneously provides a hum-free source of voltage for energizing a vacuum tube receiver-and acts as an output amplifier ofthe receiver, and

Fig. 8 is a diagram illustrating a system similar to Fig. 7, in which a pentode typeof vited, 'E is a source of voltage, which may be either. a direct current, or pulsating current source. Pulsating current is here used to indicate combined direct and alternating current such asoccurs in the unfiltered output of a rectiinclude resistors I3 and I4 in series and paralleled by aloador work circuit I5 and vacuum tube I5. The connection'of I6 is such that I4 is included in the "grid circuit and I5 in the plate circuit.

The resistors I3 and I4 areso chosen that the resistance of I3 is to the resistance of I4 as ,u is to I, where p. is the amplification constant of the vacuum tube I 6. It is 'well known, and hence need not be mathematvacuum tube. As stated in Van Der Bijl The Thermionic Vacuum Tube, page 160, the value of the grid potential necessary to reduce the current to zero is Her-ea is the amplification constant and Ep is the plate-cathode potential. Therefore in Fig. l,

where the voltage E is applied to I3 and I4 in series, these resistors divide the voltage into a voltage drop across I3, which is the grid potential, and a voltage dropvacross I4, which is the plate potential (Ep), and since the ratio of I4 to" I3 is the condition for zero plate current is obtained asin Van Der Bijls statement.

Fig. 2,'again, has the voltage source E, which is here understood to be a pulsating current source, having a large direct-current component and an appreciable alternating-current or hum component, such as would appear in the output oi 's. rectifier of alternating current.

The elements having the same functions are numbered asin Fig. l. Condenser I'I, however, is inserted .in the path or connectionof I3 and. I4 efiecwhich are present in the source E. The series I combination I3 and I'! is therefore essentially a resistance having a value n times that of resistor I4, and hence no alternating current components present in source E will flow through the load I5 and plate-cathode path of tube I6. Direct current, however, will flow through this path and will be of a magnitude determined by the voltage E, the resistance of I5, the resistance of tube I6 (as determined by the voltage drop across I4, whichthus serves as a vg1'id.bias),,a.nd the resistance of I4. I3.and I'4 aretherefore so chosen that, in addition to bearing a ratio equal to it, their values determine the bias on the grid of I6 to permit direct current to flow through I5 suflicient to operate I5 as a humless source of direct current. As will be subsequently shown, it is contemplated that I5 shall be a complete, or the major part of, a radio receiver of the vacuumtube type, requiring hum-free voltages for energizing the tubes.

The direct current voltage available from E'is divided, of course, into a voltage drop across tube I6 and the constant hum-free voltage across 15. In a practical system, then, E is designed to be sufiiciently high to provide for the drop across I5, I6 and I4, while allowing sufficient amplitude of voltage to act as an operative source for I5. Generally, it is feasible to choose values of I3 and I4 so that capacity I1 need not be a large and expensive unit. Where the rectified source E includes only a certain definite frequency component which must be eliminated, I! may be chosen of smaller capacity, and an auxiliary condenser IB, shown in dotted line, may be arranged in shunt with I4 to give this portion of the circuit the same reactance as the branch I3 and N.

Fig. 3 must be directly compared with Fig. 2 as it is an alternative arrangement and corresponding parts are similarly numbered. Here the condenser ll of Fig. 2 is omitted and instead a condenser I9 prevents the direct current voltage drop across I4 from being applied between cathode and grid of I6. Condenser I9, however, is chosen to have a low reactance relative to resistance I4 for frequencies in source E which are tobe suppressed so that the resistors I3 and I4, in the ratio of u to 1, are eefictive in balancing out hum in the plate circuit'path which includes the load I5. However, in order to predetermine the desired amount of direct current which flows in the plate circuit, a direct current bias of proper amplitude is provided and determined by tapping I4 at an appropriate distance from the cathode and leading this voltage to the grid via a high resistance 20. Resistance 20 should be high in value relative to the reactance of I9 so that it does not act as a short circuit. It will be clear, then, that here again is provided a humfree path between II and the plate of IE to serve as a source of hum-free direct current voltage to energize a load I5. As before, it is contemplated that E is the output of a rectifier of alternating current and that I5 is a vacuumtube radio receiver or the major part of one.

Referring now to Fig. 4, the circuit is essentially the same as Fig. 2, except that a screengrid type or four-element tube It is used. The screen 2| is connected to resistor I3 to provide it with proper positive potential relative to the cathode. This is possible without upsetting any of the balanced relations for eliminating hum, as previously described for Fig. 2.

Fig. 5 has all the essential elementsof Fig. 2 to provide a hum-free path or source between the plate of I6 and terminal II. Here, however, the

transformer 23, the secondary of which energizes a loudspeaker 24 indicated to beof the electro-- dynamic type in that it includes a field winding I5, which is shown in parallel with the load I5. Ithas been found that a signaling voltage, shown as E1 (which may, for example, be the audio output of a radio receiver requiring additional amplification), may be applied to the grid of I6 through 22, and thus be repeated in amplified form by I6 and 23 to operate speaker 24 without introducing any appreciable hum from source E in the speaker. Thus, the arrangement shown permits I6 to function simultaneously to provide a hum-free source of voltages andto providea stage of amplification. g

Fig. 6 shows another arrangement which is similar to that of Fig. 2 and Fig.5 in that the arrangement of I3 and I4 is such as to provide thehum-free source between H and the plates of tubes I6 and in that the tubes I6 are used for the additional functionof amplifying signals. Here, however, the tubes I6 are shown to be in a push-pull or balanced arrangement for the signals applied to input transformer 22 and outalso in phase between the cathodes and plates of the tubes through load I5. Here load I5 is assumed to be a radio receiver the output of which is applied to transformer 22 so that tubes 'I6,in addition to providing the hum-freesource for energizing I6, act as the final output amplifier of the radio receiver. One'advantage of the pushpull arrangement here is that in addition to its well-known features of reducing distortion by the balancing out of even power harmonics, the audio voltage in output transformer 23 is conjugate to the path energizing I5; hence no audio voltage is passed back into the receiver I5 and audio feed-back is avoided.

In addition to the feature of hum elimination operating differentially to amplification and the feature of feed-back balance, a further advantage of the push-pull arrangement shown is that the requirement of the ratio ,1 to 1 for resistances I3 and I4 may here be departed from to a large degree; in fact, a hum-free receiver may result with I3 and I1 omitted. .The reason for this is that hum voltages present in the source E, when applied to the push-pull tubes themselves, balance out in the output transformer 23 and hence the push-pull amplifier is itself unaffected by hum frequency components in its power supply. Furthermore, as has been indicated, the load I5 has been here definitely assumed to be the major part of a radio receiver, and its character as a load is therefore represented by capacity 26 and resistor 25.

This is a fair representation since the direct current input terminals of multi-tube receivers are always by-passed to close the radio frequency circuits and to prevent feed-back couplings through the power supply. Capacity 26 can therefore be considered as the shunt capacity element of a resistance-capacity filter. Similarly, the resistance of tubes IS in parallel can be considered as the resistive element of this resistance-capacity filter. It will be clear then that the resistancecapacity filter is an effective unit in preventing humvoltag es from being applied to the receiver I5, and-since the tubes -I6,-as:'statedffunction as amplifiersindependently of hum voltagesflin their power supply, it is not essential to preserve the bridge balance arrangement of resistances I3 and Min theratioof [.L to 1."

This last-mentioned advantage of I the tubes -I 6 and capacity 26,forming a resistance-capacity filter, is also'finherent in all of'the-otherfigures where I5, is the major part of a radiO receiVer having by-pass capacities and hencei's a general reason for not requiring an exact hum balancing' arrangementof-resistors 'I3 and;

Coming now toFig-"I, a complete radioTre-i] 28 is the primary for attachment" to the usual house-light circuit (not 'shown); 'wmuingza serves to energize'thepcathodes of *the'ivacuum tubes of radio receiver I5. Windingjfi energizes the cathode of rectifier 33 whilehighvoltagejisi applied to the plates :of 33 by 'winding3I Wind; ing 32 energizes the cathode of the amplifying and 25.. hum-eliminating tube I6. A center tap from, 3I

leads to condenser"34 which1shuntsthejoutput terminals II and I2 -to acta's a partial filter so that the rectified'voltage as developed across I l and I 2-is largely direct current 'witlian altr g nating current or frequency component." The branch I3, I'I, I4, as in'the previous figures, proportions the-voltage in the. ratio "of mtoifI-f The hum-free path from II to theeplate fof" I6 includes radioreceiver I5and output'transformer, 23, which supplies loud speaker 24. v The output of I 5 is supplied to the grid of IS via input transformer 22. Electrostatic shield 38 is interposed between the windings of 22 and connected to I2.

Thus, in operation, transformer 21, tube33, and condenser 34 act as the usual power supply to provide a direct current source of power which contains hum at theterminals II, I2. Due to the values of resistors I3'and I4, tube I6 functions to provide a hum-free source of voltage for operating radio receiver I 5. The output of I5 is applied to the grid of I6, which acts also as an amplifier supplying the loud speaker 24 via transformer 23. The economy of this arrangement will be clear when it is remembered that the usualexpensive hum-eliminating filter is omitted, and tube I6, which acts in place of the filter, also acts as the output tube of the radio receiver.

It was stated in connection with Fig. 6 that one advantage of the push-pull arrangement was that the signal was not fed back into receiver I5. While this advantage is not present in Fig. 7, described, nor in Fig. 8, which follows, yet it has been possible to. use the single tube arrangement, and with careful choice of the cir-j cuit constants to achieve large amplification by I6 without the introduction of hum feed-back I 5. Electrostatic shield 38' pleteas to'the receiver and is in fact an actual circuit lwhich hasbeen in satisfactory use.- The power supply transformer 21, having windings 28, I3; 30;}! r and 32-, and connected to rectifier 33 and condenser-34, acts exactly'as'the similarly numbered parts of the circuitof Fig, '7 in producing-direct current voltage-plus a hum frequency component at the-terminals lland I2. li-lias thescreen- 2| connected -tol3, as in A. to determine its voltage. The=hum-free pathfromterminalll to the plateof l5 includes theload [5 which is the major part-of a radio receiver, the speaker field J5 and by-pass condenser lfp" and; also the output audio transformer 23, '{WhiCh feeds the loud speaker '24. 'I'he structure-oi the receiver I5 is not of itself a partgof this invention and hence needs no detailed description other than to say that it employsthree screen-grid tubes 35, 36, 31 arranged '5 to constitute two radio frequency amplifying stages anda detector. The output of the detctor .31 is arranged to include transformer :22,

andfin order to prevent any hum component which might flowfrom terminal ll through the capacity betweenvwin'dings of transformer 22 to the control grid of. IS and hence regenerate and build up hum irithe speaker 24, there is provided an'electrostatic shield'38 grounded to terminal |z. j V a The receiver ;|5 includes between the terminals marked and of the plate circuit of IS the resistances 39, 40,, 42, whereby proper operating potentials having no hum are made available for "feeding the screens and plates and 35$ proper, grid bias to tubes 35, 36 and 31, The "speaker 'field I5 is shown included in parallel across this humless voltage source, and the by-vpass c0ndenser'|5"is also shunted'across the +-and terminals to prevent the audio output of 16 being introduced back into receiver I 5. Thus it will be seen that tube l6 provides a humless source of direct-current potentials for operating 15, and at the same time acts as an output amplifier for the receiver 15.

What is claimed is: p

1. Means for supplying a loadwith a constant potential direct current from a variable voltage source-comprising in combination a high resistance load requiring a high potential, a resisance, an amplifying vacuum tube connected in series with said load and said resistance, said resistance being included between cathode and grid of said tube, and a second resistance in series with a condenser in shunt with said load and the plate-cathode path of said tube, the ratio of said second resistance to said first mentioned resistancebeing approximately equal to the amplification constant of said vacuum tube.

2. In combination, a source of pulsating current, a path comprising a resistor, a capacity, and a second resistor in series shunting said source, a load circuit and the plate-cathodepath,

of a thermionic vacuum tube connected in series shunting said resistor and capacity, the cathode-grid path of said tube shunting said second resistor, the resistance ratio of said first 

